1. Field of the Invention
The present invention relates to a lighting device for illuminating a display element of a non-self light emission type and to a display device used in electronic equipment. In particular, the present invention relates to a liquid crystal display device used in mobile information equipment, a mobile phone, and the like and to a lighting device, such as a front light or a backlight, for illuminating the display element.
2. Description of the Related Art
A display device which has been used in a mobile phone or a mobile computer in recent years often employs a liquid crystal display device capable of obtaining a high-resolution color image with low power consumption. The liquid crystal display device employs a liquid crystal element of a non-self light emission type, and therefore the liquid crystal element is illuminated by using a lighting device having a high-intensity white LED as a light source.
Specifically, the mobile phone employs a reflective liquid crystal device which is bright and has a large aperture or another liquid crystal device capable of displaying image information on both front and back sides. A white LED used for illuminating the display elements thereof is structured such that a resin containing yellow phosphors dispersed therein is provided immediately in front of a light emitting surface of a blue LED of an InGaN system, a GaN system, or the like. With this structure, original blue light is mixed with yellow light, thereby producing white light. As regards a phosphor for converting blue light into yellow light, a YAG phosphor is well known. The yttrium aluminum garnet (YAG) phosphor is obtained by doping YAG with rare earth elements. There has also been known a method in which in stead of using the YAG phosphor, a red light emitting phosphor and a green light emitting phosphor are mixed, to thereby obtain white light through additive color mixture of blue, red, and green. As regards a phosphor for converting blue light into green light or into red light with relatively high efficiency, a chalcogenide phosphor and a phosphor nitride which are doped with rare earth are well known. There has also been disclosed an LED display device in which a plurality of light emitting elements which emit light of wavelength shorter than the wavelength of blue light are arranged on a print board of a predetermined shape and area having a circuit formed thereon, and each of the light emitting elements is covered with a translucent resin which contains a wavelength conversion material.
There has also been disclosed a method, for example, in JP 3417384 B, in which a transparent film containing a wavelength conversion material is arranged between a light guide plate of a backlight and an LCD panel, rather than potting a blue LED with a wavelength conversion material, to thereby produce white light.
When at least two kinds of phosphors each emitting light such as green light, red light, or yellow light through excitation of blue light or ultraviolet light are used together with a blue LED so as to perform additive color mixture, it is possible to attain an LCD module excellent in color reproducibility. However, according to a structure in which a red phosphor and a green phosphor are mixed to be applied onto a transparent film as disclosed in relation to a conventional technology, light emitted from the green phosphor is used to excite the red phosphor, or an in-plane particle density increases, which leads to a problem in that it is difficult to attain high luminance.
Meanwhile, according to additive mixture of two colors respectively obtained from a blue LED and a YAG phosphor (so-called “pseudo white LED” structure), light components with a wavelength range of 600 nm or more are scarce, which hinders realization of an LCD module excellent in color reproducibility. In general, according to a current color filter technology, it is regarded as being extremely difficult to exceed the NTSC ratio of 100% by using the pseudo white LED as a light source.
A conventional backlight employing a white LED is used in combination with an LCD panel. However, an optimal color balance of the backlight is different depending on the optical system of the LCD panel. It is difficult to adjust colors of the backlight for individual LCD panel by controlling an additive amount of phosphors to be potted into the LED. In the existing circumstances, the color balance of the backlight is adjusted by ranking the chromaticity of a certain block in variation of mass production. Accordingly, it is not always possible to obtain a light source of optimal chromaticity.